charmina I think you're just unlucky with your font choice in Gimp.
A quick inspection of the font in Font Forge shows that the author has placed the topmost control points of the offending letters outside Gimp's text rendering comfort zone.
The font was designed with the standard Em size (for ttf fonts) of 1000 and within that the ascender height was set to 750 (think capital letter height) - the default size for many fonts. The y co-ordinate of the top-most control point for the capital letter M has been set to 1209 by the author - a fair way above that 750 value.
Gimp does allow for quite a bit of wiggle room to render fonts - other programs may allow more space - but not quite enough for this particular font. The attchments below show you that Gimp is almost (but sadly not) there.
Your options as far as I can tell: Choose another font; email the author for a new version; try another graphics application and import it into Gimp...
Font Forge view of capital M control points:
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Magnolia_font_font_info_1.jpg [ 189.22 KiB | Viewed 3004 times ]
Same view with capital M added so you can see where Gimp's rendering almost makes it.
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Magnolia_font_font_info_2.jpg [ 170.97 KiB | Viewed 3004 times ]